Increasing the efficiency of general lighting is one of the main ways to reduce global power consumption. About 25% of the electrical power produced in US is used for lighting. Since conventional lamps are not quite inefficient, increasing the efficiency of general lighting will yield major energy-saving benefits worldwide. In the United States, the U.S. Department of Energy (DOE) estimates that power consumption for lighting could be reduced by 50% simply by replacing conventional lamps with solid state lamps.
Currently, general lighting is achieved using a wide variety of lamp types: incandescent, fluorescent, tungsten halogen, sodium, clear metal halide, etc. The most commonly used types of lamps are incandescent and fluorescent, with incandescent lamps being most widely used in residential settings. These types of lamps are available in many different forms and with different total lumen outputs and color characteristics. Two other important lighting parameters are the Color Temperature (CT) and the Color Rendering Index (CRI). The CT is a measure of the lamps “whiteness”, its yellowness or blueness, or its warmth or coolness. CRI is a measure of the quality of the light; in other words how accurately do colors appear when illuminated with the lighting source. The highest CRI is 100 and typically incandescent lamps have the highest value of CRI, as their emission spectrum is basically identical to a black body radiation spectrum. Other types of lamps may have a lower CRI. It is possible for different lamps to have the same color temperature but a different CRI.
The economics of lighting has three main variables—light (lumens, lm), power (watt, W), and cost (dollar, $). Luminous efficacy is the amount of light produced per unit of electrical power (lm/W). Purchase cost efficacy is the amount of light produced per dollar (lm/$). For reference, incandescent lamps have a luminous efficacy of about 8-15 lm/W. The purchase cost may also be referred to as the first cost.
For users of light, the cost of lighting has two components, the first (purchase and installation) costs, and the cost of the electricity (operating cost). While incandescent and fluorescent lamps have a relatively high purchase cost efficacy, their luminous efficacy and operating costs are relatively high because of their low luminous efficacy. In continuous operation, an incandescent lamp costs about $90-$120 per year and must be replaced after approximately 1000 hours (1.5 months). In normal residential use (about 14 hours/week of on time) incandescent lamps cost about $8/year to operate and must be replaced in about 18 months. Operating cost may also include the cost to change lamps, in addition to the electricity cost.
In order to reduce energy consumption associated with general lighting, new types of lamps are being investigated. One of the new types that are receiving interest because of its potentially relatively higher efficiency is a light emitting diode (LED)-based lamp. LEDs have demonstrated luminous efficacies over a wide range, from about 20 to about 140 lm/watt (here LED means a package containing an LED die; in a few cases a package may have several discrete die mounted in one package).
LEDs may emit light in one wavelength range, for example red, amber, yellow, green, blue, etc. or may be designed to emit white light. In general single color LEDs are made by using a material whose bandgap emits light in desired the wavelength range. For example, yellow, amber or red light may be produced using LEDs formed from the AlInGaP material system. Blue, UV and green LEDs may be formed, for example, using the AlInGaN material system. In other examples, single color LEDs may be formed using a combination of a LED emitting light of a first wavelength and a light conversion material, for example a phosphor, that absorbs a portion or all of the first emitted wavelength and re-emits it at a second wavelength.
White LEDs may be produced by a number of techniques, for example by combining an LED with one or more light conversion materials such as phosphors, here referred to as a phosphor-converted white LED, or by color mixing of multiple LEDs emitting different colors (a typical color mixing arrangement may comprise a red, a green and a blue LED but other combinations may be used) or by combinations of one or more phosphor-converted LEDs and one or more direct emitting LEDs, that is LEDs that do not comprise a phosphor. LEDs may have a high CRI (>90) and a warm CT, producing a quality of light similar to that of an incandescent lamp. The CT and CRI of an LED depend on the spectral output of the LED as well as the characteristics of the phosphor. Warm colors and a high CRI typically require more emission in the red wavelength range, and phosphor efficiency and the human eye's sensitivity in the red wavelength range is relatively lower than that in green-yellow range. Thus LEDs with a warm color and/or a relatively high CRI typically have a relatively lower luminous efficacy than LEDs with a cool color and/or a relatively low CRI.
Conventional LED lamps, also called prior art LED lamps, have improved luminous efficacy (about 40 to about 70 lm/W) and long lifetime (about 30,000 to about 50,000 hours) compared to incandescent lamps. However, conventional LED lamps are very expensive (about $80 to about $130 per lamp) and thus, even though the electricity costs are typically about ¼ to about ½ that of incandescent lamps, the high first cost (also known as the purchase cost) is sufficient to prevent widespread adoption. A LED lamp comprises the LEDs and any necessary electronics, optics, thermal management systems and housing to permit it to operate on generally available AC power.
Two key problems with prior art LED lamps are (1) the luminous efficacy of the LED lamp is only about 50-70% of that achievable with an individual LED and (2) the very high first cost. Although higher first costs can be mitigated by reduced operating costs and longer lifetimes, customers' price expectations often pose psychological barriers to sales. The DOE predicts that a payback time of no more than 2 years (less than two incandescent bulb lifetimes (IBL)) will be required to accelerate adoption of LED lighting, Meaningful energy conservation is possible only with widespread adoption of LED lighting. The cost element with the biggest impact on the payback time is the first cost—for conventional LED lamps it is a virtually insurmountable obstacle. For example, the current approximately $100 lamp first cost leads to a payback time of about 10 years or about 7 IBL.
Payback time may be calculated in many different ways, but two main ones are the out-of-pocket approach and amortization approach. In the out of pocket approach, payback occurs when the cost of the LED lamp plus its electricity cost equals the cost of the incandescent lamp or lamps plus its electricity cost. This calculation includes the purchase of an additional incandescent lamp at the end of the payback time, to reflect the fact that going forward one would need a new lamp after the previous incandescent lamp burned out, whereas the LED lamp would keep operating because of its significantly longer lifetime. The amortization payback time approach amortizes the lamp cost over its entire lifetime, thus resulting in shorter payback times than the out-of-pocket approach. The out-of-pocket approach may be more realistic and representative of how such decisions are made and is the calculation mainly used for payback times in this document.
The lower luminous efficacy of prior art LED lamps compared to individual LEDs is caused by a number of factors including the reduction of luminous efficacy of the individual LEDs under actual operating conditions, the cost of assembling multiple LEDs into a lamp, the optical losses associated with the use of multiple LEDs in the lamp and fixture, the efficiency losses associated with the power converter and LED driver and the further reduction in efficiency and lifetime of all components from high operation temperatures, which in turn is a result of high junction temperatures and high current densities at which LED die are operated to achieve the desired total lumen output Each of these factors will be discussed in detail below.
Very few individual LEDs produce enough light to be used individually for general lighting. In most cases LED lamps contain a relatively large number of individual packaged LEDs, on the order of about 10 to about several hundred. It is understood that a large number of individually packaged LEDs increases the lamp cost as it then includes the package cost for each LED as well as the assembly cost of putting all of the LEDs together in the lamp. Thus the current industry direction and DOE roadmap is to drive the LEDs at higher currents in order to generate more light from each LED while minimizing the required number of LEDs.
The problem with this approach is that the luminous efficacy of an individual LED decreases strongly with increasing drive current (for a given LED die size one may use a value of current and when comparing LED die of different sizes, one may choose to use either current or current density). The relatively high luminous efficacies demonstrated for prior art LEDs are achieved by operating at very low current (current density). At the low currents used to achieve the high luminous efficacy numbers, these devices produce relatively little light; certainly not enough to replace a single incandescent lamp. For example individual LEDs may produce, at these current levels, about 5 to about 200 lumens. Operation at higher current produces more light, but at the expense of significantly reduced luminous efficacy. For example, FIG. 1 shows a plot of luminous efficacy (top graph) and total light output (bottom graph) as a function of drive current for several state of the art LEDs. In particular, there are three curves in each graph of FIG. 1 showing cool white LEDs made by Lumileds (K2 star package, dash lines), Cree (XLamp XR-E, dotted lines), and Luminus Devices (SST-90, dash-dotted lines). The data for the brightest bins for each device is used. Each curve has a thick solid part which represents the driving conditions at which devices are designed to operate. It is clear that the luminous efficacy decreases rapidly with increasing current, and that the luminous efficacy within the designed operation range is about half of the peak luminous efficacy that could be achieved at low currents.
The data shown in FIG. 1 is taken with the LEDs maintained at 25° C. In actual operation maintaining such a low temperature is not possible and as the current increases, the LEDs operate at significantly higher temperatures, in the range of about 50° C. to about 100° C., which causes the luminous efficacy to drop further. Advanced packaging techniques may be utilized to improve the heat removal and slow the junction temperature rise; nevertheless, the luminous efficacy of the same LED at high currents is almost always lower than that at low currents in actual operation.
Another problem with the conventional LED lamp approach of using high currents (current density) is that the temperature rise associated with this mode of operation decreases the lifetime of an LED. The LED lifetime is generally referred to as the time period during which the total light output of an LED decreases to some percentage, typically 70%, of its initial light output level (LEDs do not typically burn out, but instead have a gradual reduction in brightness). Driving an LED at relatively high current densities and junction temperatures causes degradation mechanisms to accelerate. A specific example of this is shown in FIG. 2 taken from DOE document PNNL-SA-51901, April 2007, Thermal management of white LEDs. One can see that for a typical GaN-based LED the lifetime is about 40,000 hours when the junction temperature is about 63° C. (curve 136) and decreases to about 14,000 hours when the junction temperature is about 74° C. (curve 138). It should also be noted that as the brightness decreases, the luminous efficacy is also decreasing, so a longer-lived LED may have a higher luminous efficacy and thus may have a lower cost, over its lifetime.
Because these LEDs are designed to operate at high current, the package must be designed to manage the relatively large amount of heat (about 1 to about 5 watts) generated when the LED is operated at relatively lower luminous efficacy (at high currents). Such packages are expensive, thus reducing the purchase cost efficacy. FIG. 3 shows the luminous efficacy (top graph) and the purchase cost efficacy (bottom graph) as a function of lumen output (the lumen output is varied by changing the drive current, as shown in FIG. 1) for the same devices as in FIG. 1. The same line types are used for each device and the thick solid part of each curve represents the designed operating range. It is clear from this data that the purchase cost efficacy (lm/$) increases rapidly with increased lumen output but this is at the expense of luminous efficacy. For example, for the Luminus Devices SST-90, a purchase cost efficacy of about 40 lm/$ is only achieved at a relatively low luminous efficacy of about 60 lm/watt, about half the value achieved at very low currents. At low currents, where the luminous efficacy is about 120 lm/W, the purchase cost efficacy drops to only about 10 lm/$.
As discussed above, a prior art LED lamp contains a relatively large number of LEDs. FIG. 4 shows a schematic of an exemplary prior art LED lamp. Each packaged LED 142 is mounted on a carrier or circuit board 141 and coupled with interconnects 144. The lamp housing 146 may also contain the power converter (to convert the 120 VAC input to a DC level suitable for the LED driver and the LED driver (together shown as 148, inside housing 146) that provides a controlled current to the LEDs, appropriate thermal management systems including heat sinks 150, and an optical system 140 to combine and diffuse and/or direct the light emitted from the individual LEDs into a desired profile exiting the lamp and a base 152.
LED lamp manufacturers must purchase multiple LEDs and assemble and integrate them on the circuit board. This can be relatively expensive, and the cost increases when using higher and higher power LEDs. In addition to more expensive LED die, the LED package cost increases as well. As the LEDs are designed to operate at higher and higher temperatures, the packages for such LEDs become more complex and more expensive, in some cases costing more than the LED die itself. As discussed above, the luminous efficacy decreases at high current levels where the LEDs are designed to be operated. Thus the LED package must be designed to handle the large amounts of heat generated when the LED is operating at the relatively lower luminous efficacy of the operation point. Such packages are expensive, and the need to use multiple such packages in the lamp significantly increases the lamp cost. Associated with this are the cost of the carrier or circuit board and the cost of attaching the LED packages to the circuit board or carrier, for example using solder. The lamp cost is further increased because of the need for advanced thermal management systems required by operation at sub-optimal luminous efficacy values. Such thermal management systems are often passive, including for example metal core circuit boards, heat sinks and heat radiating fins, but in some cases may even include an active device such as a fan. All of these factors act to decrease the LED lamp purchase cost efficacy, increase the total cost/unit time and decrease the lifetime of the lamp.
Referring again to FIG. 4, one can see that the light emitted from the lamp must traverse a large number of interfaces with different refractive indices. Starting with the LED die, the light passes from the semiconductor die through the encapsulation of the package, a region of air, the lens/diffuser system 140 and then out of the lamp. Thus the light must traverse at least 4 interfaces. Each interface is associated with optical losses resulting from the difference in refractive indices of the materials forming the interface. Even at a 00 incident angle part of the incident light may be reflected back and potentially absorbed and converted to heat. The amount of the light reflected back depends on the values of refractive indices of the materials forming the interface—the closer they are, the less light that may be reflected back. At a typical semiconductor (for example Si, GaAs, GaP or GaN)/air interface up to about 30% of visible light may be reflected back, as the ratio of refractive indices between semiconductor and air is typically around 3.
Another optical loss mechanism that occurs at the interface is total internal reflection. According to Snell's law, light incident upon an interface is refracted or reflected depending on the angle of incidence and the index of refraction on either side of the interface according to the equation n1 sin θ1=n2 sin θ2 where n1 and n2 are the index of refraction on either side of the interface, θ1 is the incident angle and θ2 is the refracted angle. A schematic of this is shown in FIG. 5 in which material 1 is identified as 164, material 2 is identified as 162 and the interface between material 1 and material 2 is identified as 160. 165 identifies the normal direction to the interface, 166 represents the angle of incidence in material 1 and 168 represents the refracted angle in material 2. When the angle of incidence is large enough, no light is refracted, but instead all light is reflected back into material 1. This situation is called total internal reflection (TIR) and may be the cause of large optical losses. For example, using typical values for semiconductors and encapsulants, all light incident upon that interface at an incident angle greater than about 27° will be totally internally reflected. TIR light will suffer absorption losses within the die as it is reflected from various interfaces, leading to reduced light output and increased generation of heat. For an example of a simple rectangular LED die, only about 28% or less of the light generated within the LED die may escape the die, into the surrounding encapsulant. A great deal of work has gone into improving the light extraction efficiency of the die within the package/encapsulant. As discussed above, the LED lamp includes at least three (3) other interfaces at which Snell's law applies and at which further optical losses may occur.
Additional optical losses may also occur in lamps comprising a plurality of individual LEDs, for example packaged LEDs, arranged on a circuit board or carrier because of the increased etendue of the optical system (etendue refers to how “spread out” the light is in area and angle). In a given lamp or fixture design, one can only capture all of the light from the light source if the etendue of the light source is below a certain value (that value depends on the optical design of the lamp or fixture). In other words, as the dimensions of the light emitting area increase (the etendue increases) it becomes more difficult to focus and direct the light into a desired pattern without optical losses.
Lamps may also suffer further optical losses when put into a fixture. This is more significant for incandescent and fluorescent lamps that emit light in a relatively omnidirectional pattern. LEDs emit light in a direction pattern and thus LED lamps may suffer less light losses when installed in a fixture. Typically the optical efficiencies associated with light loss from prior art LED lamps in a fixture are in the range of about 80%.
In some embodiments the LED lamp may use 120 VAC for its input source and this may be converted to DC to drive the LEDs in a constant current mode. In this situation electronics, also called the driver, may be required to convert the 120 VAC to a DC voltage and current suitable for the LEDs.
The electronics efficiency is affected by its output voltage and typically increases as the output voltage approaches the input voltage. For example FIG. 6 shows the efficiency of a National Semiconductor LM3445 Triac LED driver as a function of output voltage for an input voltage of 115 VAC. As the output voltage increases from about 33 volts to about 47 volts (corresponding to about 10 and about 14 GaN-based LEDs in series, respectively), the driver efficiency increases from about 85% to about 90%. LEDs typically operate in the range of about 2.5 to about 4 volts and when the number of the LEDs in the lamp is relatively low, the difference in LED voltage and the input voltage is relatively large, resulting in a relatively low efficiency. The electronics efficiency may also be a function of temperature, decreasing with increasing temperature. In some embodiments, the electronics efficiency may begin to decrease when the ambient temperature rises above about 500 C. FIG. 7 shows the power conversion efficiency of an exemplary 20 watt AC/DC converter (Recom RAC20-S_D_TA series) as a function of temperature. As can be seen, the power conversion efficiency decreases significantly over about 50° C. In the typical operating temperature range for prior art LED-based lamps (about 60-C) the power conversion efficiency has decreased by almost 50% for this particular power converter.
Typically the efficiencies associated with present day electronics (drivers) are in the range of about 85%. While the electronics efficiency may be able be improved with respect to both output voltage and dissipated heat for conventional LED lamps, this may lead to unacceptable increases in the electronics form factor and cost.
As discussed above, prior-art LED lamps comprising one or more packaged LEDs driven at relatively high currents and high current densities generate significant heat because of their low luminous efficacy. As the temperature is increased, the light output of LEDs is reduced, thus further reducing the luminous efficacy. The LED and driver lifetime is also reduced as potentially is the efficiency of the driver. FIG. 8 shows a table presenting the percentage of input power that is converted to either heat (IR+conduction+convection) or light for exemplary incandescent and fluorescent lamps and exemplary prior art LED lamps. It can be seen that the LED lamps produce significantly more visible light power than incandescent lamps, but still produce a significant amount of heat which must be managed appropriately. Typically this means higher operating temperatures and more expensive packaging and thermal management techniques.
The overall luminous efficacy of a LED lamp may be expressed as where is the luminous efficacy of the LED and is a function of temperature, is the optical efficiency and is the efficiency of the driver electronics and is a function of ambient temperature. For example, the LEDs from dotted and dash-dotted curves in FIG. 1 which may have a low current luminous efficacy of about 135 lm/W, in typical prior art operation conditions may have a luminous efficacy of about 75-85 lm/W. Using values of 85% for both the optics and driver efficiency, the prior art LED lamp in operation would then have an overall luminous efficacy of 54 lm/W. Note that this does not include any temperature effects. Typical design guides recommend an 85% derating for the LED related to temperature, yielding a temperature-adjusted overall LED lamp luminous efficacy of only about 46 lm/W.
Alternatively the LED lamp efficiency may be described as the product of non-temperature sensitive factors for the LED, the optics and the driver and a temperature factor, which may also be called the thermal efficiency; ηlamp=ηLED*ηDriver*ηOptics*ηThermal. FIG. 9 shows current (2008) and 2015 target values for these four efficiencies as determined by the U.S. Department of Energy (Multi-Year Program Plan FY'09-FY'15, Solid State Lighting Research and Development, March 2009). In FIG. 9, the efficiency of the LED is expressed as a percentage, that is, the light output power divided by the input electrical power. For reference a neutral white LED efficiency of about 30% corresponds to a LED luminous efficacy of about 90 lm/W. From this table it can be seen that even though advances have been made in conventional LED lamps, the overall efficiency of conventional LED lamps is only about 17%.
In spite of all of these issues, the overall luminous efficacy of prior art LED lamps is still higher than that of incandescent lamps. However, conventional LED lamps are very expensive ($80-$120 per lamp) and thus, as discussed previously, even though the electricity costs are typically about ¼ to about ½ that of incandescent lamps, the high first cost is sufficient to prevent widespread adoption.
There is accordingly a general need in the art for techniques and devices that simultaneously provide high efficiency, high brightness and low cost in LEDs, LED lamps and LED lighting systems.